My invention pertains to ground fault detector apparatus. More particularly, the invention pertains to a vital, solid-state ground detector arrangement for detecting grounds on each terminal of a direct current source, particularly for railroad signaling systems.
In signaling systems using direct current, ground fault conditions may occur in the circuits at times, particularly where such circuits are carried in underground cables. Should the ground faults occur in the control circuits for relays, these devices may pick up or release, depending upon their connections, and function to initiate circuit operations for which they are provided. Particularly when such relays control the operation of safety devices such as railway signals or track switches in a railway signaling system, it is desirable that the ground fault condition be detected and removed as soon after occurrence as possible. In addition, it may be further desirable to inhibit the further functioning of the signaling system, or at least those particular elements where the fault condition exists, until the trouble is corrected. Detector apparatus for ground faults is known in the signaling art. However, with the advent of the use of solid-state elements in such signaling systems, a need has developed for a solid-state circuit arrangement for detecting such fault conditions whose operation is reliable and vital so that improper operation of the signaling system may be inhibited until the fault is corrected.
Accordingly, an object of my invention is an improved ground detector apparatus using solid-state elements.
Another object of the invention is an improved ground detector for a local direct current source which independently detects positive and negative fault currents even if simultaneous ground faults occur.
A further object of the invention is vital ground detector apparatus for a railroad signaling system which alternately checks the presence of ground faults on the positive and negative leads from the direct current energy source, registering any detected fault condition by the release of a normally energized vital relay.
Still another object of my invention is detector apparatus which alternately compares the level of ground fault currents from opposite terminals of a direct current energy source against alternately selected reference current levels to determine if a ground fault condition exists on either energy source terminal.
Another object of the invention is solid-state ground detector apparatus for a railroad signaling system which holds a vital registry relay energized as long as alternate comparisons of the level of the ground leakage current from opposite terminals of the direct current source against preselected reference current levels determine that each leakage current is less than the associated reference current, detection of the opposite current condition from either source terminal or both actuating the release of the relay to register a ground fault condition.
A further object of my invention is a solid-state ground detector for a direct current source in which a switching means and a current pump element are driven to alternately produce associated signal pairs, each comprising a ground fault current and a selected level reference current, successive pairs relating to the opposite terminals of the direct current source; the successive pairs of current signals are applied with opposing relationship to a comparator which produces successive output pulses each indicating in accordance with the relative levels of the corresponding signal pair whether or not a fault condition exists; and which output is supplied through a synchronizing network to hold a vital relay in a no-fault indication condition as long as the comparator output when synchronized with the input signal pairs indicates the absence of any ground fault on either terminal of the source.
Other objects, features, and advantages of the invention will become apparent from the following specification and appended claims when taken in connection with the accompanying drawings.